Cardiac performance is commonly conceptualized in terms of cardiac output (CO). This value is found by multiplying left ventricular stroke volume by heart rate (CO=SV x HR). However, different sized individuals have different cardiac outputs, so the preferred measure is cardiac index (CI), calculated by dividing cardiac output by body surface area (BSA); that is CI=CO/BSA. If patient’s height and weight are known, their body surface area (in m2) can be obtained using the Dubois surface chart. The normal cardiac index is 2.8-3.6 L/minute/m2.

Factors affecting cardiac outputi. The blood volume available for ejection – the venous return or preload.ii. The resistance to ejection – the afterload.iii. The strength of ventricular myocardial contractility(Ventricular preload, afterload and contractility together determine the stroke volume).iv. The heart rate and rhythm.

The continuous measurement of cardiac output can be performed using one of the following methods:1. Continuous thermodilution cardiac out put2. PiCCO3. LiDCO4. NiCCO5. Doppler techniques6. Thoracic Bioimpedance

CONTINUOUS CARDIAC OUTPUT USING THERMODILUTION TECHNIQUE

Continuous Cardiac output Catheter features:Continuous cardiac output (CCO) is measured by the thermodilution method using a modified pulmonary artery catheter. The continuous cardiac output (CCO) catheter is similar to a standard thermodilution Pulmonary artery catheter; certain models include a venous infusion (VIP­) lumen. This flow directed pulmonary artery catheter is designed to monitor hemodynamic pressures and to provide for continuous and bolus measurement of cardiac output. It has the following features;

Swan-Ganz Continuous Cardiac Output Thermodilution Catheter

a. Proximal injectate lumen: The blue lumen, or proximal injectate lumen terminates at a port located 26 cm from the distal tip of the catheter. When the distal tip is correctly positioned within the pulmonary artery, the proximal injectate port will reside in the right atrium, allowing for bolus cardiac output injections, right atrium pressure monitoring, infusion of IV solutions and blood sampling.b. VIP Lumen: If present, the clear VIP lumen terminates at a port located 30 cm from the distal tip of the catheter. This port allows for infusion of IV solutions, right atrial pressure monitoring and blood sampling.c. Distal Lumen: The yellow distal lumen terminates at the distal tip of the catheter. During insertion, this port is used monitor catheter location, via transitional pressure measurements, as the catheter is advanced forward. At full insertion, this port will reside in the pulmonary artery, allowing for measurement of pulmonary artery pressure and mixed venous blood sampling.d. Balloon inflation valve: The red balloon inflation lumen terminates in a latex balloon at the distal tip of the catheter. When the catheter is properly positioned in the in the pulmonary artery, the balloon is inflated intermittently for the measurement of pulmonary artery wedge pressure (PAWP). The balloon is inflated by syringe, with air or CO2.e. Thermistor Lumen: The thermistor lumen contains the electrical leads for the thermistor bead, which is positioned at the catheter surface 4 cm from the distal tip of the catheter. The thermistor is used to measure pulmonary artery blood temperature and generates the thermal curve, which is used to calculate cardiac output.f. Thermal filament: The thermal filament is 10 cm in length and is located between 14-25 cm from the distal tip of the catheter. When positioned correctly within the heart, it lies between the RA and the RV. The thermal filament emits as energy signal, which is used to cardiac output continuously.Continuous cardiac output methodology:

Swan-Ganz Thermodilution Catheter in placeIt is now possible to obtain cardiac output at the bedside on a continuous basis. The same general principles of thermodilution that apply to intermittent bolus are used to measure continuous cardiac output. The difference is the indicator that is used. The intermittent bolus technique uses a cool, fluid injectate as the indicator. CCO technology uses small energy impulses (warming of blood) that are emitted directly into the blood stream as the indicator. Unlike the intermittent bolus technique, no fluid bolus is required. Small energy impulses are emitted directly into the blood via the thermal filament in a random on-off pattern. When the thermal filament is in the “on” sequence, the surface temperature of the catheter is increased by approximately 4-7O C. This random on-off pattern (pseudo-random-binary sequence) is repeated very 30-60 seconds. The subsequent change in blood temperature is measured by the thermistor, which lies in the pulmonary artery. The overall increase in blood temperature sensed at the thermistor in the pulmonary artery is typically less than 0.05o C above the baseline blood temperature. The vigilance monitors internal algorithm cross correlates the input signal with the resultant change in temperature measured by the thermistor, and a “wash-out” curve is generated. CCO is calculated from the area beneath the curve.

The striking advantages of the Pulsion PiCCO are:1. Continuous pulse contour monitoring2. Volumetric monitoring3. Flexible use without a pulmonary artery catheter4. Less invasive than a pulmonary artery catheter5. Real beat-to-beat signal6. Short response time7. Also applicable in paediatric patients8. Rapidly set up and easily used9. Reduction of intensive care costs

Lithium dilution Cardiac output (LiDCO)The LiDCO / pulse CO device represents a combination of a simple indicator dilution technique used to calibrate an arterial waveform analysis algorithm. Theoretically this combination should provide beat-by-beat measurement of cardiac output, with little clinical increment of risk, assuming that in these critically ill patients arterial and venous lines would already be in situ. The LiDCOTM/PulseCOTM system. Blood is sampled from the arterial line via the three-way tap in the manometer line.

The use of lithium as an alternative indicator for the estimation of cardiac output was first described in 1993 and has now been extensively validated. In brief, isotonic lithium chloride (150 mM) is injected as bolus (0.002-0.004 mmol/kg) via the central, or peripheral, venous route and a concentration – time curve generated by an ion-selective electrode attached to the arterial line manometer system. The cardiac output is calculated from the lithium dose and the area under the concentration – time curve prior to re circulation using equation1:Cardiac output = Lithium dose (mmol) x 60 / Area x (1-PCV) (mmol/second);where the area is the integral of the primary curve, and PCV is packed cell volume (Hb (g/dl).

Non invasive Cardiac output (NICO)NICO is a non-invasive cardiac output monitor from Novametrix. It uses a method know as partial CO­2 re-breathing, which is based on the well-accepted Fick principle. With this method the cardiac output is proportional to the change in CO2 elimination divided by the resulting in end tide CO2. These changes are accomplished and measured by the proprietary NICO sensor, which periodically adds a re-breathing volume into the breathing circuit.

DOPPLER TECHNIQUESa. Pulmonary artery catheter DopplerA PAC (no longer commercially available) that incorporates an ultrasonic transducer has been developed. The catheter is curved in such a way as to maintain contact with the wall of the PA. Using the Doppler principle, instantaneous SV is calculated from the mean velocity of blood flow in the main. The accuracy of this technique was favorable compared with an electromagnetic flow probe when tested in a compared well with the Fick and thermodilution methods in patients during cardiac catheterization.

b. Transtracheal DopplerDoppler CO may be determined transtracheally. The equipment consists of a 5 – mm ultrasonic transducer bonded to the distal end of an endotracheal tube. The shape of the cuff is ellipsoidal to ensure contact between the transducer and the anterolateral wall of the trachea. The technique is similar to transesophageal Doppler techniques in that the calculation of CO is based on approximations of aortic cross-sectional area (CSA), the angle of incidence between the ultrasound beam and direction of blood flow within the aorta, the integral of the systolic velocity-time curve, and the heart rate. The theoretical advantages of this method over the transesophageal approach are that measurement of blood flow is in the ascending aorta (proximal to the arch vessels), which allows for the more constant anatomic relationship between the trachea and the ascending aorta. Disadvantages include the potential for compromising ventilation during positioning of the ultrasonic probe, the possibility that the probe might damage airway structures, and frequent manipulations to maintain correct probe placement.

The technique correlated well with intermittent thermodilution CO determinations in both animal and human studies. The transtracheal Doppler CO monitor revealed a small underestimation of the CO, appeared to perform much better in patients whose Doppler signal required minimal manipulation of the probe, and had a high correlation with the ability to track trends in CO. In another study there was not such a high correlation between transtracheal Doppler CO and intermittent thermodilution Co measurements. The authors point out that this may have been secondary to the investigator’s experience and the fact that the patients enrolled were undergoing cardio thoracic surgery where manipulation of the aorta may interfere with the transtracheal probe.

c. Transesophageal and Suprasternal Doppler:Specialized Doppler probes may also be placed in the suprasternal notch to interrogate the ascending aorta or in the esophagus adjacent to the descending aorta. Again, the flow within the vessel is proportional to the integral of the systolic velocity-time curve, the aortic CSA, and the heart rate. A number of studies have found good correlations among the aortic ultrasound techniques and other CO monitoring systems. There remains, however, a question whether the degree of accuracy is sufficient when favorable clinical reports are critically examined using the method of bland a Altman statistical analysis. Limitations of these systems include the need for frequent probe repositioning, decreased accuracy during aortic manipulation, and the calibration procedures. Furthermore, Kamal et al demonstrated that the esophageal Doppler CO monitor tended to be inconsistent during periods of acute blood loss.

TRANSESOPHAGEAL ECHOCARDIOGRAPHY

Echocardiography may also be used for the measurement of CO by measuring flow through the heart valves. Using TEE, the velocity-time integral of flow through the mitral valve is multiplied by the calculated valve area and constant. This is then multiplied by HR to determine the CO. While the calculations are time-consuming at present, the degree of accuracy has been promising.

OTHER TECHNIQUESPulse ContourAnother method of CO measurement is the aortic pulse contour analysis. This technique requires a central aortic catheter and makes assumptions concerning the distensibility of the systemic arterial bed that may not be valid with wide variations in SVR. The notion that SV can be quantified from the pulse pressure dates back to observations by Erlanger and Hooker in 1904. In a critical review of the numerous pulse pressure contour methods of measuring CO, Kouchoukos et al found an overall correlation coefficient of 0.928 with a standard error of the estimate of 17.4 percent for Warner’ method. Weissman et al were able to show that arterial pulse contour was able to trend CO correctly in patients subjected to esmolol induced hypo tension and phenylephriene-induced hypertension. Using a noninvasive technique to measure pulse contour and continuous CO with the Finapres device, Gratz et al observed a modest correlation with this device compared to the intermittent thermodilution method(r=0.75, p=<0.01)>

THORACIC BIOIMPEDANCEThe first attempts at measuring CO by thoracic electrical impedance date back to 1966, when Kubicek et al presented an empiric equation for the calculation of left ventricular stroke volume. To measure thoracic electrical impedance, and alternating current of low amplitude and high frequency is introduced and simultaneously sensed by two sets of electrodes placed around the neck and xyphoid process. Changes in thoracic impedance are induced by ventilation and pulsatile blood flow. For the measurement of SV, only the cardiac-induced pulsatile component of the total change in electrical impedance is analyzed (dZ/dt) as the respiratory component is filtered out.

Donovan et al compared CO measurements obtained by transthoracic impedance and by thermodilution in 27 critically ill patients, using the standard Kubicek equation. They were unable to find a satisfactory correlation between the two CO methods. Bernstein’s modification of the Kubicek equation was evaluated in critically ill patients. This improved the overall correlation with intermittent thermodilution CO(r=0.88), and 85 percent of the data points fell within 20 percent of the intermittent CO. The greatest disparity between the two techniques was observed at very low flows (<2l/min).>